YOU dont wann use a 85 atc250r swingarm. you would want to use 86+ the 85's have a crappy carrier design, and are very espensive to replace. you can only use 85 stuff in the 85 arm.
86+ will have a brake caliper set up. what im gonna do on my 250r is replace it with 06+ 450r. its 1" longer then and 04-05 450r swingarm. same lenght as 85/86 250r swingarms.
your also forgetting 300ex didnt have prolink, you had a no link shock/swingarm. a 250r has a linkage and so does a 450r.
everynow and then on exrides a +2 comes forsale for like $150 bucks, its well worth it.
or if ya wanna simplify all this all my buddy hreatv.com and have what ever lenght u want made, his prices are very fair.